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INFLUENCE OF THE NITROGEN CONTENT ON
THE INTERNAL FRICTION OF Fe-16.6 wt % Cr
ALLOYS
M. Lebienvenu, B. Dubois
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C5, suppte'ment au nO1O, Tome 42, octobre 1981 page c5-91 I
INFLUENCE OF THE NITROGEN CONTENT ON THE INTERNAL FRICTION OF
Fe-16.6 wt % Cr ALLOYS
M. Lebienvenu and
B.
DuboisLaboratoire de Me'taZZurgie, E.N.S.C.P., 11,
rme
Pierre e t Marie M e , 75231Paris
~ e d e x 05, FranceAbstract.- Using N2-Hz m i x t u r e s , c o n t r o l l e d q u a n t i t i e s o f n i t r o g e n were i n t r o - duced i n pure iron-chromium a l l o y s . The i n t e r n a l f r i c t i o n was measured d u r i n g h e a t i n g and c o o l i n g on a 1100°C as quenched sample between room temperature and 400°C. A maximum Q-&, was detected as soon as the n i t r o g e n c o n t e n t was i n t h e 130-200. w t % range. A t a frequency v = 0.9 Hz, the temperature o f t h i s maximum Tm was s l i g h t l y decreased from 270°C t o 250°C when t h e n i t r o - gen c o n t e n t was i n c r e a s i n g .
A
martensi t i c phase appeared i n the l l O O ° C as quenched a l l o y f o r a n i t r o g e n c o n t e n t between 480 and 580.10-4%.
Then an increase of t h e h e i g h t o f t h e maximum and a m o d i f i c a t i o n o f Tm were detected. Below a 5 8 0 . 1 0 - ~ w t % n i t r o g e n content, t h e maximum was broad and a small r e v e r s i b l e peak was detected d u r i n g c o o l i n g . Comparing these r e s u l t s w i t h those obtained on s i m i l a r a l l o y s c o n t a i n i n g carbon, t h e behaviour o f t h e s e t w o i n t e r s t i ti a1 s i n f e r r i ti c i ron-chromi um are d i soussed.1. I n t r o d u c t i o n . - The c a r b u r i z a t i o n o f a pure f e r r i t i c iron-chromium was used t o show t h a t a maximum a t 553 K i n the as quenched a l l o y occured b e f o r e t h e appearance of a m a r t e n s i t i c phase. I n t h e same way pure iron-chromium a l l o y s were n i t r i d e d t o know the e f f e c t o f n i t r o g e n on t h e ( a
+
y ) a boundary o f t h e iron-chromium a t l l O O ° C , b u t a l s o t o i n v e s t i g a t e t h e behaviour o f n i t r o g e n d u r i n g the tempering o f l l O O ° C and800°C as quenched a l l o y s .2. Experimental procedure.
-
A pure i ron-chromium was e l a b o r a t e d i n our l a b o r a t o r y . The chemical a n a l y s i s y i e l d e d C r = 16.6 w t % whereas r a d i o a c t i v a t i o n a n a l y s i s i n d i - cated C = 0 . 7 . 1 0 - ~ w t % and N = 7 . 1 0 - ~ %. A f t e r c o l d r o l l i n g,
p l a t e s (79 mm x 7 mm x 0.5 mm) were maintained one hour e i t h e r a t l l O O ° C o r a t 800°C on argon atmosphere b e f o r e quenchin9 i n b r i n e . The dampin? c a p a c i t yQ-l
was measured on an i n v e r t e d t o r - s i o n pendulum o s c i l l a t i n g a t v = 0.9Hz,
the shear amplitude was E = 2.10-5. Theh e a t i n q r a t e was 1 2 0 " ~ . h - I from room temperature up t o 400°C.
Pure samples were n i tri ded under N2-H2 m i x t u r e s i n s u i t a b l e e q u i l i b r i um c o n d i t i o n s t o tntroduce c o n t r o l l e d n i t r o p e n q u a n t i t e s i n s o l i d s o l u t i o n i n the a1 l o y . For the s t u d i e d a l l o y the t r u e n i t r o g e n s o l u b i l i t y were r e s p e c t i v e l y 0.016 w t % a t 800°C, 0.023 w t % a t 900°C
,
0.031 w t % a t 1000°C and 0.042 w t % a t 1100°C ( 2 ) .3. Experimental r e s u l t s . -
3.1. I n t e r n a l f r i c t i o n o f the pure a l l o y
-
Whatever the quenching temperature (1100°C o r 800°C) i s a f l a t i n t e r n a l f r i c t i o n spectrum was obtained. The i n t e r n a lC5-912 JOURNAL DE PHYSIQUE
f r i c t i o n was s l i g h t l y decreasing w i t h t h e temperature and t h i s background was n o t i n f l u e n c e d by the vacancy c o n c e n t r a t i o n ( 3 ) .
3.2. I n t e r n a l f r i c t i o n o f n i t r i d e d a l l o y s
-
The 1100°C as quenched a l l o y s showed an i n t e r n a l f r i c t i o n maximum a t 280°C (553 K) o n l y when t h e n i t r o g e n content reached 204. % ( f i g u r e 1 ) . A t lower concentration, the background was aboutQ-I
= 8 . 1 0 - ~ . This maximum was i n c r e a s i n g l i n e a r l y w i t h t h e n i t r o g e n c o n t e n t up t o 421.( f i g u r e 2 ) . However the temperature o f the maximum Tm was s l i g h t l y decreased ( f r o m
Nitnded ~ e l 6 . 6 ~ r ( w t % ~ x l 0 ~ ) as 1100°C quenched
a =
0.95Hz € = 2 .m-S/\
J. \L
I I I T-c - F i q .I
: I n t e r n a l f r i c t i o n as a f o n c t i o no
100 200 300 m ~ e r a t u r e f o r t h e as quenched Fe-16.6 % C r a l l o y s .553 K t o 538 K ( f i g u r e 2 ) . The as quenched s t r u c t u r e as the 400°C tempered micro- s t r u c t u r e was a simple phase. During c o o l i n g a s m a l l e r maximum was detected and
=I+
on t e h e i g h t o f i n t e r n a l f r i c t i o n : The e f f e c t o f n i t r o g e n content peaks.was observed a f t e r 18 h a t 40OoC. The p r e c i p i t a t e was i d e n t i f i e d by X r a y d i f f r a c t i o n as the C r N n i t r i d e ( f i g u r e 3 ) . F i g . 3 : M i c r o s t r u c t u r e developed d u r i n g X i n g (18 hours) a t 400°C f o r a quenched a1 l o y (Fe,16 % Cr, 0.0421 % N) G x 600
I f t h e n i t r o g e n c o n t e n t was h i g h e r than-421. %, a martensi t i c phase ( f i g u r e 4) and a p r e c i p i t a t i o n were detected by e l e c t r o n microscopy i n the as quenched s t a t e ( f i g u r e 5 ) . Simultaneously t h e i n t e r n a l f r i c t i o n was increased d u r i n g tempering and t h e temperature o f t h e t h i g h t maximum was m o d i f i e d w i t h o u t t o r e l a t e t h i s temperature
9 Sf*
r .
.*a-
Jy"ib
Fig. 4 : O p t i c a l micrography o f ah
+ IP?.
7.,t.TaLIT
C .;e.,
.
-
-7 C r 0.0583 % N quenched a l l o y
1
,
3 from 1100°C.%*\
, XI I, 6 x 600w i t h t h e temperatures d e t e c t e d before.
C5-914 JOURNAL DE PHYSIQUE
a f e r r i t i c s t r u c t u r e w i t h a p r e c i p i t a t i o n o f CrN. During tempering a maximum o f i n t e r n a l f r i c t i o n was observed o n l y i f the n i t r o g e n content was up t o 4 2 1 . 1 0 - ~ % and even w i t h 5 8 3 . 1 0 - ~ % o f n i t r o g e n , t h e maximum was lower than those observed before. 4. Discussion.
-
An essay i s made t o compare o u r r e s u l t s w i t h those obtained i n t h e case o f t h e c a r b u r i z a t i o n o f a pure i r o n 16.9 % chromium a l l o y ( 1 ) . The n i t r o g e n atom s i z e i s s m a l l e r than the carbon atom s i z e whereas the e l e c t r o n e g a t i v i t y of n i t r o g e n i s h i g h e r than the carbon e l e c t r o n e g a t i v i t y . The b i n d i n g energy Cr-N i s h i g h e r than those o f C r - C an i t i s p o s s i b l e t o suggest a tendancy o f n i t r o g e n atom t o s t a y i n i n t e r s t i t i a l s i t e s r a t h e r than t o m i g r a t e towards l a t t i c e defects. I t has been shown by Jack (4) t h a t i n t e r s t i t i a l n i t r o g e n i n t h e body-centred cubic l a t t i c e causes a d i s t o r s i o n . This can be b e s t v i s u a l i s e d by c o n s i d e r i n g an octahedron o f i r o n atoms. N i t r o g e n occupies t h e i n t e r s t i c e a t the center o f t h i s octahedron. The r e s u l t i n g d i s t o r s i o n c o n s i s t s o f an expansion along the t e t r a d a x i s and a c o n t r a c t i o n along the o t h e r axes. Chromium atoms s i t t i n g s u b s t i t u t i o n a l l y i n the i r o n l a t t i c e c o n t r a c t t h e l a t t i c e . Thus i f a chromium atom occupies one o f t h e s i t e s o f an i r o n octa- hedron, along the t e t r a d axes, then the energy o f t h i s system, w i t h an i n t e r s t i t i a l n i t r o g e n atom a t the center, would be l e s s than f o r any octahedron c o n t a i n i n g no chromium atom. The expansion r e q u i r e d by t h e i n t e r s t i t i a l n i t r o g e n atom i s p a r t i a l l y compensated by the c o n t r a c t i o n r e q u i r e d by t h e chromi um atom. The d i s t o r s i o n presen- t e d t o the surrounding l a t t i c e i s reduced by t h e presence o f chromium. Thus the n i - trogen atoms w i l l p r e f e r t o s i t i n octohedra o f which the t e t r a d axes c o n t a i n chro- m i um atoms ( f i g u r e 6 ) . Considering s p e c i f i c a l l y the i n t e r n a l f r i c t i o n mechanism,Octahedral sites in ( a ) p u r e alpha i r o n F i g . 6 : Octahedral s i t e i n the
[b)pure alpha i r o n p l u s n i t r o g e n
-
.
n i t r o g e n v ~ c ~ n i t y o f a chromium atom. 0 i r o n (c)chromium i r o n plus n i t r o s e nQ chromium
under an a1 t e r n a t i n g s t r e s s the n i t r o g e n atoms w i l l jump i n t o e q u i v a l e n t s i t e s around t h e chromium atoms. The a c t i v a t i o n energy correspondin? t o jumps f o r these s i t e s w i l l be l e s s than f o r Fe s i t e s due t o t h e energy gained by a l s o accomodating a chromium atom. This p o i n t o f view was a l r e a d y proposed ( 5 ) o r denied ( 6 ) .
a p r e c i p i t a t i o n was never observed up t o a n i t r o g e n content equal t o 5 8 3 . 1 0 - ~ % n e i t h e r i n the quenched a l l o y nor i n t h e 400°C tempered a1 l o y : 18 hours were r e q u i - r e d t o o b t a i n C r N p r e c i p i t a t i o n i n the a l l o y w i t h 421.10-4 % o f n i t r o g e n . B u t an a1 l o y w i t h 583. % o f n i t r o g e n showed simultaneously martensi t e and Cr-N p r e c i p i - t a t e s i n the quenched s t a t e .
The maximum d u r i n g tempering was broader than i n Fe-Cr-C and i t s temperature was depending on the amount o f n i t r o g e n . I t seems t o us t h a t several phenomena were present : i n t e r a c t i o n s w i t h l a t t i c e defects (quenched-in vacancies and d i s l o c a t i o n s ) and jumps from an i n t e r s t i t i a l s i t e t o another e q u i v a l e n t s i t e . Thermodynamic s t u d i e s p e r m i t us t o develop a theory about the s i t e occupancy b u t t h i s t h e o r y was n o t p r e - sented here ( 2 ) . I n t h e same. way works are i n progress t o i n t r o d u c e the p a r t o f t h e Snoek Kb;ster theory i n t h e observed phenomenon. To solve these two d i f f i c u l t i e s can e x p l a i n t h e reason o f a steady maximum appearing i n m a r t e n s i t i c a l l o y s .
4 4.2. 800°C as quenched a l l o y s
-
True n i t r o g e n s o l u b i l i t y was 160.10- % a t 800°C and t h e as quenched a l l o y was a f e r r i t e and C r N p r e c i p i t a t e s . A small maximum appeared i n t h i s a1 l o y o n l y when t h e n i t r o g e n c o n t e n t was 421. % and i t seems t o us t h a t i t i s r e l a t e d t o a s u r s a t u r a t i o n o f the m a t r i x , necessary t o observe a maximum. I n these c o n d i t i o n s i t i s d i f f i c u l t t o o b t a i n p r e c i s e i n f o r m a t i o n about t h e l a t t i c e d e f e c t s and c l u s t e r s .5. Conclusion.- The i n t e r n a l f r i c t i o n due t o n i t r o g e n atoms i n a pure f e r r i t i c Fe- 16.6 % C r was studied. A f t e r quenching from 1100°C a maximum was observed a t about 553 K o n l y i f t h e n i t r o g e n content was i n t h e 130-220.10-~ % range. Any s t r u c t u r a l phase changes were detected n e i t h e r a f t e r quenching nor a f t e r tempering. This i s an i m p o r t a n t d i f f e r e n c e w i t h t h e Fe-Cr-C system. The observed maximum would be r e l a t e d t o i n t e r a c t i o n w i t h l a t t i c e d e f e c t s b u t also w i t h jumps o f n i t r o g e n between equiva- l e n t i n t e r s t i t i a l s i t e s . When t h e n i t r o g e n content was 5 8 3 . 1 0 - ~ %, t h e as quenched a1 l o y was a martensi t e w i t h C r N p r e c i p i t a t e s : the i n t e r n a l f r i c t i o n maximum was s t e a d i e r and higher. Moreover i t appears a t a temperature s l i g h t l y d i f f e r e n t and i t was d i f f i c u l t t o g i v e now a complete model o f these phenomena.
References
1. !.I. BOUHAFS, B. DUBOIS, 111 European Conference I.F.U.A.S., Manchester, J u l y 1979, Ed. C.C. Smith, Pergamon Press (1980) ,343.
2. M. LEBIENVENU, Thesis Doct.Sci. (1981), U n i v e r s i t y P a r i s 6. 3. M.A. KRISTHAL, V . I . BARANOVA, Phys.Met.Metallo., 12, 137 (1961). 4. K.H. JACK
